In general, an orthogonal frequency division multiplexing (OFDM) is a scheme suitable for high speed data transmission in wired and wireless channels, and is currently adopted as a transmission scheme for several high speed communication systems. That is, when a single subcarrier scheme is used when high speed data having a short symbol period is transmitted in a wireless communication channel, inter-symbol interference is more serious, and thus complexity of a receiving end is significantly increased. However, when a multi-subcarrier scheme is used, a symbol period in each subcarrier can be expanded as many as a number of subcarriers while a data transmission rate is still maintained, so that a simple equalizer having one tap can effectively deal with a serious frequency selective fading channel due to a multi-path.
Since the OFDM scheme uses a plurality of subcarriers having the orthogonality, the efficiency in using a frequency is increased, and processes of modulating and demodulating the plurality of subcarriers by a transmitting end have the same results as those generated by performing an inverse discrete Fourier transform (IDFT) and a discrete Fourier transform (DFT), respectively, and thus the OFDM scheme can be implemented using an inverse fast Fourier transform (IFFT) and a fast Fourier transform (FFT) at high speed. Such an OFDM scheme is adopted as a standard scheme in several communication systems since the OFDM scheme is suitable for high speed data transmission.
Meanwhile, an OFDM signal having passed through a multi-path fading channel is affected by a frequency selective channel in a frequency domain. Accordingly, for stable channel estimation, a pilot signal is generally transmitted to a position of a particular subcarrier so as to adopt a change in a channel in the frequency domain. At this time, an interval between respective pilot signals is designed in consideration of a size of a delay spread of the multi-path fading channel. If the size of the delay spread is increased and the change in the channel within a determined pilot interval is increased, a channel estimation error is generated, thereby causing serious demodulation capability deterioration. In order to minimize the capability deterioration, characteristics of the multi-path fading channel should be known in the process of estimating the channel, and information on the delay spread of the channel plays an important role among the characteristics.
Meanwhile, an OFDM receiver using a pilot signal carried on a particular subcarrier at regular intervals directly performs initial channel estimation only in a position of the pilot signal, and channel estimation for a data signal between pilot signals is performed through an interpolation scheme and the like using channel estimation values of the pilot signals. Initial channel estimation values in pilot positions are used for estimation of the delay spread of the multi-path fading channel, and the accuracy of the interpolation scheme may be increased through an estimated spread delay value.